Generalize parts of post_rof_mod to handle fields with ungridded dims

[billsacks]

Description of changes

This will be needed for the water tracer work. I haven't yet generalized the code for removing negative runoff; that will be done later.

(This is similar to the changes in #628 but now for post_rof_mod.)

Specific notes

Contributors other than yourself, if any:

CMEPS Issues Fixed (include github issue #):

Are changes expected to change answers? (specify if bfb, different at roundoff, more substantial) - no, bfb

Any User Interface Changes (namelist or namelist defaults changes)? No

Testing performed

Please describe the tests along with the target model and machine(s)
If possible, please also added hashes that were used in the testing

In the context of cesm3_0_alpha08e, ran:

ERS_Ly7.f09_g17_gris4.T1850Gg.derecho_intel
SMS_Lm13.f10_f10_mg37.I1850Clm50SpG.derecho_intel
MULTINOAIS_Ly2.f10_f10_ais8gris4_mg37.I1850Clm50SpRsGag.derecho_intel.cism-change_params
SMS_D_Ld1.ne30pg3_t232.I1850Clm50BgcSpinup.derecho_intel.clm-cplhist
SMS_Ld40.TL319_t232.C_JRA.derecho_intel
ERS_D_Ld3.ne30pg3_t232.B1850C_LTso.derecho_intel.allactive-defaultio
ERI.ne30pg3_t232.B1850C_LTso.derecho_intel.allactive-defaultio
ERC_D_Ln9.ne30pg3_ne30pg3_mt232.FHISTC_LTso.derecho_intel.cam-outfrq9s
ERI.TL319_t232.G_JRA.derecho_intel.cice-default
SMS_D.TL319_t232.G_JRA_RYF.derecho_intel

All tests passed and were bit-for-bit, though with these failures that also failed in the baselines:

    FAIL ERI.ne30pg3_t232.B1850C_LTso.derecho_intel.allactive-defaultio COMPARE_branch_hybrid
    FAIL ERI.ne30pg3_t232.B1850C_LTso.derecho_intel.allactive-defaultio COMPARE_rest_branch
    FAIL ERS_D_Ld3.ne30pg3_t232.B1850C_LTso.derecho_intel.allactive-defaultio COMPARE_base_rest